The present invention is intended to measure various geometric, positional and kinematic parameters with respect to and during the rotation of a rotatable device, such as a disk or drum, about a rotation axis where it is desired to assure that certain geometric, positional and kinematic anomalies in the rotation of the device are determined so that appropriate action, if any, may be taken. The present invention is particularly applicable to computerized axial tomography (CAT) scan systems and will therefore be described particularly in that context, but should not be considered to be limited to such systems.
CAT scan systems typically include a gantry formed of a structure such as a disk or drum rotatable within a frame held in a yoke. In third generation CAT scanners an X-ray source and X-ray detector array are mounted on the disk for rotational motion therewith about a table on which a patient can repose. The X-ray source and X-ray detector array are positioned about a point on the disk that defines the locus, hereinafter referred to as the "geometric center", about which the source and detector array prescribe correct rotational movement when the disk is rotated about the point during a scan so that the tomographic image can be accurately reconstructed. This geometric center ideally coincides with the nominal center of mass of the disk as well as the rotational center of the disk. In fourth generation CAT scanners the X-ray source is mounted on a rotatable disk relative to the geometric center, while the detectors are disposed on the stationary frame equiangularly about the rotation axis of the disk. In both types of systems, the X-ray source may provide periodic pulses or continuous wave radiation. Each detector typically is either a solid state or a gas tube device.
In third generation type machines the detector array is disposed diametrically opposite the source across the disk, and is aligned with the focal spot of the source so that the detector array and focal spot are positioned within a common mean, scanning or rotation plane (normal to the axis of rotation of the disk). In the case of a detector array, each detector of the array is positioned in the scanning plane, typically at a predetermined angular spacing relative to the source so that each detector subtends an equal angle relative to the focal spot, thus providing a plurality of different X-ray paths in the scanning plane between the source and the respective detectors. In third generation machines, the X-ray paths can collectively resemble a fan with the apex of the fan at the focal spot of the X-ray source. In fourth generation machines the X-ray paths with respect to each detector resembles a fan with the apex at the detector input. Accordingly, both types of machines are sometimes referred to as "fan beam" tomography systems.
These systems provide a plurality of information or data signals corresponding to variations in the radiation flux measured by the detectors at each of a plurality of projection views, i.e., at precise angular positions of the disk during rotation of the disk about an object occupying the space between the detectors and the X-ray source. Upon known (Radon) mathematical processing of the signals commonly referred to as "back projection", a visual image can be formed representing a two-dimensional slice along the plane of rotation, i.e., the scanning plane, through the portion of the scanned object positioned in the plane between the source and the detectors. The accurate formation of such images critically depends upon various factors including: (1) the movement of the disk being rotational precisely about the geometric center of the disk, (2) the geometric center remaining fixed in the scanning plane during a scan so that it does not move laterally within the plane relative to the scanned object as the disk rotates about its axis, (3) the X-ray exposure provided during each projection view being the same for each view, and (4) data being taken at precise angular positions of the disk so that data is correlated with the correct positional information of the X-ray source and/or the detectors relative to the scanned object when the image is back projected. Thus, one source of errors is due to vibrations and mechanical noise as the disk rotates within the gantry frame. Because during a scan even minor lateral movement of the geometric center of the disk of the CAT scan apparatus relative to the scanned object can cause errors resulting in faulty or erroneous images, such apparatus has been provided as massively reinforced, expensive devices often weighing a ton or more to prevent improper movement of the source and detector system.
Many of the disadvantages inherent in such a massive, expensive CAT scan structure characteristic of the prior art have been recognized and addressed, at least in part, by the apparatus described and claimed in U.S. Pat. No. 4,928,283 issued May 22, 1990 to B. M. Gordon (and U.S. Pat. No. Re. 34,379 issued Sep. 14, 1993, to B. M. Gordon) and in U.S. Pat. No. 5,109,397 issued Apr. 28, 1992 to B. M. Gordon, et al.
The former patent, U.S. Pat. No. 4,928,283 describes a lighter weight machine and some advantages it provides over the heavier machines. However, making the machine, and in particular the rotating drum, much lighter in weight creates problems for which the prior art machines were deliberately design to minimize, i.e., undesirable lateral movement and/or misalignment of the components relative to the scanned object during a scan. For example, because the X-ray source (in third and fourth generation machines) and the X-ray detector array (in third generation machines) are usually all precisely mounted relative to the geometric center of the disk, and it is assumed that the object being scanned remains fixed relative to the rotation axis during the scan, if the geometric center in the scanning plane and the center of rotation (defined as the intersection of the rotation axis and the scanning plane) are not precisely coincident, the geometric center will rotate around the center of rotation resulting in undesirable lateral movement of the components within the scanning plane relative to the scanned object during a scan. Similarly, if the center of rotation and geometric center (even if coincident) move within the scanning plane during a scan, as for example when the disk is subject to vibration, or is driven by a wheel or roller which is out of round with respect to its rotation axis, or the disk itself is out of round and driven by a wheel or roller, the components on the disk will also move laterally within the scanning plane.
The above-identified U.S. Pat. No. 5,109,397 describes, inter alia, the use of sensors that detect the proximity of the periphery of the rotating disk to the inner circular surface of a "perfect" ring disposed around the disk in the plane of rotation. The sensors then provide compensating electrical signals for modifying or correcting the data received by the x-ray detector array in accordance with deviations in the clearance between the disk and the ring. The basic rationale of this latter patent is that it is much less expensive to make the ring round within specified tolerances than to make the disk equally round. Nevertheless, the manufacture of the ring is still subject to tolerances.
In a CAT scan system, it thus is highly desirable that any lateral movement of the geometric center relative to the scanned object be kept at a very small fraction of the resolution (typically in the order of 1 mm) of the scanner, e.g. 1/10to 1/20of the resolution or less than about 0.1 mm. Where the disk in a scanner is typically five or six feet in diameter, the desired tolerance is then about one part in five million and is extremely difficult if not impossible to achieve. Using extreme and very expensive measures, machines have been built with machined disks and precision bearings to achieve accuracies on the order of 0.5 mm. However, this increases the cost of the machine.
In addition imaging errors can result (a) if the data is collected at incorrect angular positions of the rotating disk, which can be the result of inaccurate measurement of the actual angular position of the disk when each set of data for a projection view is taken, or (b) if the angular velocity of the disk varies throughout the scan such that the X-ray exposure for each of the projection views of the scan is not uniformly constant.